Electroluminescent lamps generally comprise a layer of phosphor material, such as a doped zinc sulphide powder, between two electrodes. It is usual for at least one electrode to be composed of a transparent material, such as indium tin oxide (ITO), provided on a transparent substrate, such as a polyester or polyethylene terephthalate (PET) film. The lamp may be formed by depositing electrode layers and phosphor layers onto the substrate, for example by screen printing, in which case opaque electrodes may be formed from conductive, for example silver-loaded, inks. Examples of electroluminescent devices are described in WO 00/72638 and WO 99/55121.
An electroluminescent lamp of the general type described above is illuminated by applying an alternating voltage of an appropriate frequency between the electrodes of the lamp to excite the phosphor. Commonly, the phosphors used in electroluminescent lamps require a voltage of a few hundred volts. Typically, such electroluminescent lamps may have a capacitance in the range 100 pF to 1 μF.
The inventors have been involved in the development of electroluminescent displays which comprise electroluminescent lamps having selectively illuminable regions for displaying information. Such displays have the advantage that they can be large, flexible and relatively inexpensive. In the context of such electroluminescent displays, the inventors have sought to provide a simple power supply arrangement for an electroluminescent lamp or display.
A known type of circuit for producing a higher output voltage from a low voltage DC supply is a “flyback converter”. Such a circuit comprises an inductor and an oscillating switch arranged in series. In parallel with the oscillating switch, a diode and a capacitor are arranged in series. The switch oscillates between an open state and a closed state. In the closed state, a current flows from the DC supply through the inductor and the switch. When the switch is opened, the current path is interrupted, but the magnetic field associated with the inductor forces the current to keep flowing. The inductor therefore forces the current to flow through the diode to charge the capacitor. The diode prevents the capacitor discharging while the switch is closed. The capacitor can therefore be charged to a voltage which is higher than the DC supply voltage, and current at this voltage can be drawn from the capacitor.
In order to supply an alternating current to a load from a flyback converter, an H-bridge may be provided in parallel with the capacitor. In general, an H-bridge comprises two parallel limbs, each limb having a first switch in series with a second switch. On each limb between the first and second switches there is a node, and the load is connected between the respective nodes of the limbs. Current can flow through the load in one direction via the first switch of one limb and the second switch of the other limb and in the other direction via the other two switches. The switches of the H-bridge are operated so that current flows through the load first in one direction and then in the other.
When an H-bridge is used to supply a capacitive load CL with a supply voltage V, during the first half of the cycle of operation, the load CL is at +V. When the H-bridge switches and reverses the polarity of the load, there is a potential difference of −2V between the supply voltage and the load. The load is supplied rapidly with current from the supply until there is no potential difference, and this requires 2CLV2 of energy. Similarly, when the H-bridge is switched to return the load to the original polarity at the end of the cycle, a further 2CLV2 of energy is required to bring the load back to +V.
It will be seen, therefore, that each cycle of the operation of the H-bridge requires 4CLV2 of energy. The power consumption, assuming 100% efficiency, is 4CLV2f, where f is the cycling frequency of the H-bridge. This represents a significant power consumption when the frequency and the voltage are large.
It is usual to provide a large smoothing capacitor (such as the capacitor of the flyback converter described above) in parallel with the H-bridge in order to provide current for the rapid charging and discharging of the capacitive load. The smoothing capacitor protects the power supply from the large currents which result from the switching of the polarity of the H-bridge, and ensures that the supply voltage does not drop significantly.
The inventors have realised, however, that if the smoothing capacitor is chosen to be smaller than the capacitive load, when the polarity of the H-bridge is switched, the current drawn from the smoothing capacitor by the capacitive load fully discharges the smoothing capacitor and the high voltage supply collapses. In this case, almost immediately after the H-bridge has been switched, the current supplied to the capacitive load is drawn directly from the low voltage DC supply, rather than from a store of charge at high voltage in a large smoothing capacitor.